1. Field of the Invention
The present invention generally concerns magnetic resonance tomography (MRT) as used in medicine for the examination of patients. The present invention in particular concerns a magnetic resonance tomography apparatus as well as a method for operation of such an magnetic resonance tomography apparatus, with which a high contrast (i.e. a contrast for representation of the blood circulation state of the heart wall) can be achieved in the imaging of various heart phases on the basis of a gradient echo sequence.
2. Description of the Prior Art
MRT is based on the physical phenomenon of magnetic resonance and has been successfully used as an imaging modality for over 15 years in medicine and biophysics. In this examination modality, the subject is exposed to a strong, constant magnetic field. The nuclear spins of the atoms in the subject, which were previously randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” nuclear spins to a specific oscillation. In MRT, this oscillation generates the actual measurement signal, which is acquired by suitable reception coils. By the use of inhomogeneous magnetic fields generated by gradient coils, the measurement subject can be spatially coded in all three spatial directions. The method allows a free selection of the slice to be imaged. Slice images of the human body thus can be acquired in all directions. MRT as a tomographic modality in medical diagnostics is distinguished predominantly as a “non-invasive” examination method with a versatile contrast possibility. Due to the excellent representation capability of the soft tissue, MRT has developed into a method superior in many ways to x-ray computed tomography (CT). MRT today is based on the application of spin echo and gradient echo sequences that enable an excellent image quality with measurement times in the range of seconds to minutes.
The continuous technical development of the components of MRT amplitude-phase set and the introduction of faster imaging sequences have made more fields available for the use of MRT in medicine. Real-time imaging for supporting minimally-invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples.
The acquisition of the data in MRT occurs in k-space (frequency domain). The k-space trajectory determines the sampling, i.e. the order of the data acquisition in k-space. The MRT image in the image domain is linked with the MRT data in k-space via a Fourier transformation. The spatial coding of the subject which spans k-space can ensue in various ways, but a Cartesian or a radial (per-projection) sampling are most conventional. The coding ensues by means of gradients in all three spatial directions. Given Cartesian sampling, a differentiation is made between the slice selection (establishes an acquisition slice in the subject, for example the z-axis), the frequency coding (establishes a direction in the slice, for example the x-axis) and the phase coding (establishes the second dimension within the slice, typically the y-axis). Depending on the combination or interleaving (nesting) of the three gradients in an imaging sequence, the sampling of k-space can ensue in a Cartesian manner (line-by-line) or radially or helically.
In order to measure a slice of the subject to be examined in a Cartesian manner, an imaging sequence is repeated N times for various values of the phase coding gradient (for example Gy is repeated). The frequency of the magnetic resonance signal is sampled, digitized and stored N times at equidistant time intervals Δt in each sequence pass via a Δt-clocked ADC (analog-digital converter) in the presence of the readout gradient Gx. In this manner a number matrix (matrix in k-space or k-matrix) is generated line-by-line with N×N data points (a symmetrical matrix with N×N points is only one example; asymmetrical matrixes can also be generated). An MR image of the slice in question with a resolution of N×N pixels can be directly reconstructed from this data set via a Fourier transformation.
FIG. 7 schematically shows the excitation and gradient scheme of the known FLASH (fast low angle shot) sequence. This is based on the principle of the gradient echo technique. Fast image sequences that are based on the principle of the small angle excitation and in which the echo signal is generated exclusively via gradient reversal are designated as gradient echo sequences (GE sequences). Given the small angle excitation, flip angles of α<90° are used, with only a small fraction of the longitudinal magnetization being rotated in the transversal plane. The wait for relaxation of the magnetization is thereby shorter, which leads to significant time savings. Furthermore, due to the pole reversal of the slice-selection and frequency coding gradients, the dephasing of the transverse magnetization caused by the two gradients is compensated such that a gradient echo arises. The RF pulse with a small angle excitation at an angle α is shown in FIG. 7 in the first line and the RF (radio-frequency) signal with the gradient echo is subsequently shown on the time axis. The slice-selection gradient Gz is plotted along the time in the second line. As already explained, the slice-selection gradient is superimposed on the homogeneous magnetic field during the RF pulse along the z-axis and the polarity of said slice-selection gradient is subsequently reversed for the purposes of dephasing. In the fourth line the frequency coding gradient Gx is shown along the time axis. For the frequency coding, a gradient field in the x-direction is superimposed on the homogeneous magnetic field after reversing the polarity of the gradient during the acquisition of the RF signal. The phase coding gradient Gy is shown along the time axis in the third line. For phase coding along the y-axis, a constant gradient is activated for a defined time before acquisition of the RF signal and the sequence is repeated Ny times. After the data acquisition the transverse magnetization is dismantled again by spoiler gradients activated after acquisition of the RF signal. The repetition time TR is the time for a sequence pass between two RF pulses.
FIG. 8 shows the basic principle of the True FISP sequence (fast imaging with steady precision), which is very similar to the FLASH sequence. In contrast to the FLASH sequence, in the True FISP sequence the remaining transversal magnetization is not negated by spoiler gradients after the data readout but rather is completely rephased by switching of gradients in the reversed direction along all three coordinate axes. Due to the rephasing there is a further signal contribution that is available in the subsequent RF excitations.
The exposure of fast image series is by default acquired with the True FISP sequence, since an optimal signal-noise ratio (SNR) can hereby be achieved. The image contrast depends on the ratio of the longitudinal relaxation time T1 to the transversal relaxation time T2. Dependent on the tissue regions to be acquired, the sequence is acquired with T1- or T2-weighted images; for example, T1-weighted image sequences are used for the acquisition of images of the heart or of the heart muscle. T1-weighted image sequences are in particular used to show scars and to show the blood circulation state of tissue. It is disadvantageous, however, that a good T1 contrast requires a long wait time and measurement time since no sufficient longitudinal relaxation has yet occurred directly after the inversion pulse. This problem is counteracted by implementing a magnetization preparation of the tissue before the sequence, the tissue being prepared dependent on T1 and/or T2. The image contrast varies with the interval from the preparation phase; after each preparation phase a data acquisition is therefore possible only within a short time span, which distinctly reduces the quantity of the data that can be acquired. In order to acquire a series of fast images with magnetization preparation, long measurement times are therefore required, or a poor temporal or spatial resolution must be accepted.
The problem of acquiring fast image series of various heart phases and therewith a movie (cine' representation) of the heart movement was previously solved by only a segment of the data necessary for an image being acquired, but this was acquired multiple times, and the segments from various heartbeats were respectively individually associated with a fixed heart phase. This method is disadvantageous in cases where the acquisition time is limited by a breath hold of the person to be examined, since in this case the resolution cannot be improved by decreasing the size of the acquired segments.